Halftone color separations are used in reprography as copy masters for preparing offset or relief printing plates. Before the printing plates are exposed, the color separations are checked in color proofing processes to determine whether the ultimate printing result will be a tonally accurate reproduction of the original. According to general standard specifications, for example, high quality offset printing requires a resolution of 2%-98% dots, even 1%-99% dots on modern printing machines, for a 60 lines/cm screen. It is especially difficult to achieve satisfactory resolution of the small 2% dots in the highlights and 98% dots in the shadows. For good tonal reproduction, it is especially important that screen dots of the same size be sharply delineated over the entire area and reproduced uniformly, that is, without size variations.
A considerable proportion of these color proofs is supplied by proofing presses, essentially small offset printing presses for the smallest runs. U.S. Pat. No. 5,380,620 discloses a process that uses special, photosensitive, aqueous-developable materials and can be used for color proofing processes. These materials have areas, formed by special auxiliary layers, that accept or repel printing inks, analogous to dry offset plates. Washoff processes have the important disadvantage that waste water containing chemicals must be disposed of, and additional developing processors must be used. U.S. Pat. No. 5,380,620 does mention briefly a material that can be used without aqueous development. However, these offset materials must use very special, fluorine-containing, diazo compounds to make areas that accept or repel printing inks. Tonable photopolymerizable materials are not disclosed.
These are known in other color proofing processes using photosensitive recording materials in which differences in the tackiness of exposed and unexposed areas of the photosensitive layer are used to make the image. German Patent 12 10 321, U.S. Pat. Nos. 3,620,726, 3,582,327, 3,649,268, 4,356,253, 4,948,704 and 4,243,741 disclose a reproduction process, wherein a tacky, photopolymerizable, recording material, comprises a support and a photopolymerizable layer containing at least one addition-polymerizable monomer, and a photopolymerization initiator. The recording material is hardened by imagewise exposure, whereupon the exposed image areas lose their tackiness. The latent image is made visible by the application of suitable toners that adhere only to the unexposed tacky areas and can be removed from the exposed nontacky areas. This process yields positive, optionally color, images of the original similar in appearance to images produced by the use of printing inks.
The toners, comprising predominantly finely divided powders, can be applied by being dusted on the imagewise exposed surface. According to another embodiment, the toner can also be loosely bonded on a special support and transferred by bringing this support into contact with the imagewise exposed layer. Such transfer layers are significantly more advantageous, because handling is considerably simpler, cleaner, and more friendly to the environment. Color proofing processes using pigmented transfer coatings are disclosed in German Patents 12 05 117 and 29 49 462, U.S. Pat. Nos. 4,806,451, 4,902,363, 4,939,029, 5,126,226, 5,210,001, 5,090,774, and German Patent 41 20 793, and in U.S. Pat. No. 4,935,331.
However, the cited photographic color proofing processes have the disadvantage that the preparation of multiple copies of the color separations is time-consuming and costly. New photosensitive material must be exposed imagewise and toned to obtain each copy and multiple photosensitive materials must be processed for polychromatic images. Another disadvantage is that the finished color proofs contain photopolymerized and unphotopolymerized material, which can lead to alterations in colors and changes in dot growth.
U.S. Pat. No. 3,060,025 and EP-C 0 034 816 disclose the preparation of multiple copies of a color separation by transfer of an imagewise color layer onto a receptor. However, these processes also have disadvantages. U.S. Pat. No. 3,060,025 does indeed mention the possibility of making multiple copies by the use of only one photosensitive material, but the final image also contains photosensitive starting material. The process of EP-C 0 034 816 uses toners that become tacky when heated and retain this tackiness a while after cooling, so that a portion of the toner layer can be transferred repeatedly. However, polychromatic images are difficult to prepare this way, because part of the transferred toner layer is transferred back onto the preceding color layer when a new color is applied.
Partial transfer of toner layers also alters colors. As contact exposure of the photosensitive material is not possible, because specular images would usually be obtained, the quality required for color proofs cannot be achieved with this process. This problem can be circumvented by the use of temporary supports, as described in U.S. Pat. No. 5,240,810, but the final proof contains additional adhesive layers that af quality.
The complex shear moduli of known photopolymerizable layers utilized in proofing applications, for example CROMALIN.RTM. 4BX and CROMALIN.RTM. 8BX, are outside the range of moduli described below.
The problem involved in the invention is to make available multiple, right-reading copies of a color separation or a polychromatic image without using a time-consuming and costly process, while avoiding the indicated disadvantages of known processes, and maintaining constant from the first to the last copy the color locus and resolution required for the color proof.
This problem is solved by the process described herein.